The invention relates generally to meter prover apparatus and particularly to interchanges used in such apparatus for receiving a sphere from the meter prover loop, and for relaunching the sphere into the loop.
U.S. Pat. No. 3,911,724 daTed Oct. 14, 1975 discloses meter prover apparatus incorporating an interchange which receives a sphere from the outlet end of the metering loop and causes it to be launched into the inlet end of the loop to commence a meter prover run. It consists of a shuttle assembly disposed within the interchange body and movable between sphere receiving and sphere launching positions. When in sphere launching position, means is provided for establishing a seal which prevents flow through the interchange. In addition to incorporating sealing means and a sphere carrier, the assembly is provided with means which prevents substantial backflow of liquid through the interchange when the shuttle assembly is in sphere receiving position. An improved interchange of the same type is disclosed in U.S. Pat. No. 3,998,089 dated Dec. 21, 1976. The improvement consists in providing backflow preventing means which establishes an interchange seal when the shuttle assembly is in sphere receiving position. Another feature is the means which guides the shuttle assembly to prevent misalignment. In the operation of interchanges of the type disclosed in the above patents, it has been found that under certain conditions hydraulic shock occurs when the shuttle assembly is moved from sphere receiving to launching positions. This shock appears to be due to abrupt sealing action of the sealing means carried by the shuttle assembly as the assembly is moved toward launching position. It tends to be more intense for higher flow rates and for interchanges made in the larger sizes. Such shocks are considered to be detrimental, particularly in that associated equipment components may be injured, and injury may occur to associated pipe connections.